Solar and wind generation of electricity are commercialized and growing steadily, with new installations for exceeding 50 GW/year. Efficient technologies for storage and deployment are required to adapt the temporal nature of these sources to match consumption and ensure their continued expansion. In addition, meeting the increasing demands of advanced consumer electronics and electric vehicles requires next generation rechargeable batteries with greater specific-energy and energy density than current lithium (Li) ion batteries (LIBs). Lithium ion batteries (LIBs) are a well-developed energy storage media, but they are approaching intrinsic performance limits and cannot meet the demands required for electric vehicles (EVs), advanced consumer electronics, and stationary storage.
Leading candidates to supplant LIBs include solid state batteries and those based on the lithium-sulfur (Li—S) redox pair. Solid state batteries employ conventional electrode materials (i.e. graphite/LiCoO2), but may achieve 2-3 times higher energy density than LIBs by replacing the volatile and flammable liquid electrolyte with a solid electrolyte (SE), making them competitive with Li-air batteries. Sulfide ceramics derived from lithium sulfide (Li2S) are the leading SE materials owing to their high conductivity and superior ductility, with leading candidates based on the Li2S—P2S5 and Li2S—GeS2 systems.
Li—S batteries are particularly attractive for EV applications, due to their high practical specific energy, long driving distance, and low pack prices. Room temperature Na—S (sodium-sulfur) batteries, which have similar working principle as Li—S batteries, are also highly attractive for stationary storage due to the abundance and low cost of Na. The simplest configuration for M-S batteries employs a M (M=Li and Na) anode and S cathode, but the use of alkali metal sulfide (M2S, M=Li and Na) cathodes present several advantages. First, M2S cathodes can be paired with metal-free anodes, such as existing anodes (graphite) or emerging materials (Si and Sn). For example, the practical specific energy of Si—Li2S (930 Wh/kg) is close to that of Li—S batteries (1000 Wh/kg). Second, M2S cathodes are fully lithiated, not requiring preset void space around M2S particles for accommodating the detrimental volume fluctuations that occur during charging/discharging cycles. Third, the electrode fabrication is simplified due to the greater thermal stability of M2S, and it allows batteries to be assembled in the discharged state, a safer and more cost-effective process. Emerging solid state battery architectures incorporate M2S into all three components: cathode, anode and electrolyte, demonstrating the great importance of M2S for solid state batteries.
M2S is commercially produced by carbothermal reduction processes. The high temperatures employed in these processes yield M2S in the form of micropowders, and impurities are a major concern. Additionally, Na2S is mainly commercially available in its hydrate form (Na2S.xH2O, x˜3) and contains polysulfide impurities. At present, there are no domestic suppliers, putting significant constraints on battery manufacturers that would like to integrate M2S into their products. In addition, M2S micropowders confront some shared challenges with conventional Li—S technology. First, M2S in the bulk form is a very poor electronic and ionic conductor, requiring the use of large overpotentials to initiate cycling. Second, charging/discharging proceeds through polysulfide intermediates (M2Sn, n=4-8) which have high solubility in conventional electrolyte solutions and migrate between the cathode and the anode. This can lead to a serious loss of active material and anode corrosion.
To address these issues the scientific community has invested significant effort in recent years to develop nanoparticle (NP)-based cathodes. Compared with bulk materials, the high specific surface area of NPs enables higher cycling stability, specific capacity, and rate capability. In addition, the high activation potential required for bulk materials is not needed for NPs. Nanostructure is also important for M2S-based solid electrolytes. The fabrication of the electrolyte ceramics involves mixing and annealing (200-600° C.) the constituent powders (M2S, P2S5, and GeS2).
The most common strategy for the M2S—NP synthesis is to convert commercial M2S micro-powders into M2S—NPs through high energy ball milling, which is time consuming and can introduce impurities. Other methods demonstrated in academia include recrystallization of dissolved M2S, electrochemical lithiation/sodiation of sulfur NPs, or carbothermal reduction using molecular precursors. The challenges for all of these approaches are that they are energy intensive, time consuming, and not amenable to scale-up. In addition, they are limited with respect to their level of control over size, uniformity, or morphology.
Disclosed herein are methods and systems for producing M2S nanoparticles and hierarchical structures comprising the nanoparticles (M2S—HSs). The disclosed nanoparticles, methods, and systems solve many of problem associated with existing technologies, and do so in efficient and economical ways. Regarding synthesis of M2S—HSs, the disclosed method may comprise two steps: synthesis of M2S secondary clusters (M2S SCs) and carbon coating the SCs. The former is a chemical reaction between a sulfur-containing inorganic compound and an alkali metal organic compound in an organic solution. The reaction directly generates the wanted M2S—SCs powder, which can be collected as a precipitate from the reaction solution. The latter is to integrate a carbon-scaffold with M2S SCs by carbonizing a double-shell polymer coating